Cycles Toon shader size effects diffuse direct value
According to the code currently here, and the sample_uniform_cone currently here, without smoothing the resulting diffuse intensity depends on the size of the toon shader node.

So we can compensate this value to invert the calculation:

The size is multiplied by pi/2 (so this is relative to half cone angle).

Then take 1 minus the cosinus of the previous result.

Then divide pi/2 by the previous result.

Last, multiply the intensity value by it.

As this last step is multiplication factor in the shader code, we need to divide by it in the nodes. This is done by this node group:



The result is not exactly perfect (should be some rounding or an error by me). But we obtain this: